The present invention relates to an optical element configured to reflect ultraviolet (UV) radiation at an operating wavelength below 250 nm, preferably at 193 nm, comprising: a substrate, and a reflective layer made of aluminum superimposed on the substrate, wherein the reflective aluminum layer is not transparent to the UV radiation. The invention also relates to an optical element configured to reflect UV radiation at an operating wavelength below 250 nm, preferably at 193 nm, comprising: a substrate, a reflective layer superimposed on the substrate made from a basic material which is not transparent to the UV radiation, the basic material preferably being metallic or semi-conductive, and a dielectric multi-layer system superimposed on the reflective layer, wherein the reflective optical element has a reflectivity of more than 85%, preferably of more than 88%, and even more preferably of more than 92%, over at least a range of incident angles of 10°, preferably of 15°. The invention also relates to optical arrangements for microlithography having an optical element of this kind, and to methods for manufacturing such an optical element.
Optical elements for the reflection of UV radiation (typically between 120 nm and 250 nm) are used, for example, in projection exposure apparatus for microlithography for beam deflection and/or folding of the beam path. Such optical elements are intended to make the reflectivity for incident radiation as high as possible over a range of incident angles that is as wide as possible. Usually, a dielectric multi-layer system is superimposed on a reflective layer consisting of a basic material, the multi-layer system being adapted to increase the reflectivity of the optical element due to interference effects. For example, the use of aluminum, silver, gold or platinum as a basic material for optical elements reflecting in the visual wavelength range is known. For the UV and/or VUV wavelength range usually only aluminum is used as a basic material since in other metals, as a rule, the plasma edge is in the wavelength range above the radiation used.
From U.S. Pat. No. 7,033,679 B2 it is known to select as a basic material a single-crystalline metallic layer with a high package density for an optical element reflective in the visual wavelength range, of which the surface roughness is very small and onto which a multi-layer system with enhanced reflectivity is superimposed. In the case of basic materials other than aluminum, e.g. chromium, however, the reflectivity of the optical element is distinctly below 90% once the wavelength of the radiation used is approaching the UV wavelength range.
U.S. Pat. No. 6,956,694 B2 discloses reflective layers which are used in a catadioptric projection lens for UV radiation. Among others, aluminum has been specified as a basic material for these layers having a reflectivity of more than 90% in this wavelength range. Moreover, molybdenum, tungsten and chromium have been specified as materials for these layers, the reflectivity in the UV range of which, however, is only at approximately 60%.
The reflectivity of an optical element does not only depend on the characteristics of the basic material. The reflectivity may even be enhanced if, as described in the applicant's WO 2006/053705 A1, a layer of tantalum which has good heat conductivity and is assumed to enable an oriented growth of the aluminum layer is deposited between the aluminum layer and the subjacent substrate in the case of a dielectrically protected metallic mirror.
Further to achieving a degree of reflection as high as possible other factors are still relevant concerning the quality of a reflective optical element. According to general knowledge, a difference in reflectivity and a phase difference depending on the incident angle generally occurs between radiation impinging on the reflective surface with an electrical field intensity vector parallel to the incident plane (p-polarized radiation), and radiation in which the field intensity vector is perpendicular to the incident plane (s-polarized radiation). Both the difference in reflectivity and the phase difference ought to be as small as possible across the range of incident angles of interest, since both of them may deteriorate the image-forming properties of the optical system, in which the reflective optical element is arranged, if no appropriate action is taken to compensate these effects. In deflective mirrors, the range of incident angles of interest typically is in an angular range of about 45°, the width thereof is varying in dependence of the optical design used, e.g., between 40° and 50° or between 35° and 55°.
An optical element of the aforementioned type for reflection of UV radiation which is assumed to have little fluctuation in polarization reflectivity over a wide range of incident angles is known from U.S. Pat. No. 6,310,905 B1. Therein a specific sequence of single layers is used which layers are superimposed on an aluminum layer serving as the basic material, in order to achieve a variation as small as possible of the reflectivity over the range of incident angles used.
EP 1767978A1 discloses an optical system comprising at least one deflective mirror. Radiation impinges over a wide range of incident angles onto this mirror. Upon reflection the at least one deflective mirror produces a change in the phase difference between s- and p-polarized radiation of a maximum of 30° over the total range of incident angles. Besides aluminum, silver, silicon, germanium, molybdenum and ruthenium as well have been identified as a basic material for the deflective mirror. Typically, three to four dielectric layers are superimposed onto the basic material.
U.S. Pat. No. 7,331,695 B2 discloses a visible light-reflecting member which uses a plate or a film for reflecting visible light, whose reflective surface is provided with an aluminum layer having high purity and (111) as the main plane orientation. The (111)-plane oriented aluminum layer is supposed to have increased reflectivity for visible light compared to amorphous aluminum layers. The (111) plane orientation is generated by sputtering using a bias voltage.
However, when coating a substrate with an aluminum layer for UV wavelengths, only a very limited range of coating parameters (very low pressure, high coating rate, low coating temperature) is available to achieve optimum results. Specifically, the roughness of the aluminum layer rises with a rise in the coating temperature, oxidation, and when using ion- or plasma assisted coating techniques, whereby the stray light portion increases and the reflection decreases. Thus, forming an aluminum layer by sputtering—as described in U.S. Pat. No. 7,331,695 B2—is not a viable option for producing an aluminum layer for UV/VUV applications, as the reflectivity of the aluminum layer thus produced is too low.
It was also discovered that upon application of high-power laser radiation the aluminum layer is roughened even when said layer is protected, e.g. by a protective layer consisting of chiolithe or a dielectric multiple-layer system.
Moreover, the inventors have observed that upon superimposing a dielectric multi-layer system onto the aluminum layer with a single layer or plural layers being superimposed through coating processes involving high energy the roughness of the aluminum surface does not decrease as is the case with other basic materials, but on the contrary, frequently increases.